The present invention is related to wireless communication, and in particular, to frequency synthesizer architecture suitable for multi-band radio transmitters (and receivers) that have a fixed intermediate frequency (IF).
Radio transmitters that use a classic superheterodyne architecture can only cover a radio frequency (RF) bandwidth that is a small fraction of the first intermediate frequency (IF), unless tunable RF filters are used after the second upconversion to RF, or there is extremely high isolation from the local oscillator (LO) to RF.
FIG. 1 shows in simplified form a typical superheterodyne transmitter architecture 100 that includes a first upconverter 103 to convert baseband (or bandpass) I and Q signals to the first IF, an IF amplifier 195 and a filter 107 in the IF path, a second upconverter 109 to convert the filtered IF to RF, and a power amplifier (PA) 111 to amplify the RF. The amplified RF is coupled to a transmit antenna. The two upconverters 103 and 109 use signals from a low-side local oscillator 113 and a high-side local oscillator 115, respectively.
One alternative to using tunable RF filter between the RF upconverter 109 and the antenna is to have an extremely high isolation from the local oscillator(s) to RF. Another alternative is to use a zero-IF architecture, also called direct conversion and homodyne. There are however known advantages to using a superheterodyne architecture over a zero-IF architecture.
It is desired therefore to use a superheterodyne architecture. It also is desired to not use a tunable RF filter. An alternative to a tunable RF filter is to use a superheterodyne architecture, but with a variable IF frequency. A variable IF frequency however is not always easy to achieve or desirable.
One application of interest—and one for which variable IF frequency is a problem—is a transceiver for a wireless local area network (WLAN) such as a WLAN that conforms to one of the IEEE 802.11 standards. Two frequency bands of interest are the 2.4 GHz band (the ISM band) used for example for the IEEE 802.11b and 802.11g standards, and the approximately 5 GHz bands (up to 5.825 GHz) for the IEEE 802.11a and European HiperLAN standards. The level of performance needed in a WLAN typically means that the IF path in a superheterodyne transceiver for WLAN applications is typically a high quality discrete filter, e.g., a discrete SAW filter. Variable IF frequency is usually precluded with such filters.
Is it desired to build substantially monolithic integrated circuit that implements a superheterodyne transmitter and receiver that operate in both these disparate WLAN RF bands. Thus there is a need for a frequency synthesizer that generates the local oscillator signal for a superheterodyne radio transmitter and receiver that has fixed IF and that can be used for both the 2.4 Hz and approximately 5 GHz WLAN bands.
It is further desired to use a fixed IF that is a sizable fraction of the tuning range. For example, an IF of 770 MHz is a sizable fraction of the tuning bandwidth of the approximately 5 GHz WLAN bands. It further is desired to have the same IF path for both bands, e.g., the same transmit front-ends that generate the IF signal for both RF bands. It is also desired to have a relatively low tuning range required for any signal controlled oscillator device, e.g., a voltage controlled oscillator (VCO) in the frequency synthesizer. It also is desired to use a single frequency synthesizer to provide both local oscillator signals for both the 2.4 GHz and approximately 5 GHz RF bands.
Thus there is a need for a frequency synthesizer that generates the local oscillator signal for a superheterodyne radio transmitter and receiver that has these desirable properties.